1. Field of the Invention
The present invention relates to dobbies for controlling the heddle frames thereby ensuring formation of the shed in weaving looms.
2. History of the Related Art
The majority of textile dobbies, whether they are the rotary type or use pivoting levers, are known to be arranged to operate with an open shed, in that, during each cycle, they actuate the heddle frames so that the frames are moved in a total vertical stroke, without stopping at a median position corresponding to the closure of the shed. However, in certain cases, and more particularly for weaving long-length tubular fabrics, heavy dobbies operating with closed shed must be employed. The heddle frames in such case are controlled to move from the median position of closure in a direction of one or the other of of closure in a direction of one or the other of the extreme positions of opening.
The heavy closed-shed dobbies marketed at the present time generally include the structural elements schematically illustrated in FIG. 1 of the accompanying drawings. For each heddle frame a, the mechanism includes an arm b which is coupled to the frame by a drawing system c associated with a rod system or other connecting device, opposite its pivot point d, each arm b is provided with a double hook e of the swinging type, controlled at its ends by a reading device f containing the weaving program. With the arm b are associated two independent knives g which are actuated to move away from and towards each other simultaneously, on either side of a median position for which they are applied against the free end of arm b.
Under these conditions, it will be readily understood that the reading device f controls the double hook e to thereby connect the arm b with one or the other of the two knives g. The heddle frame a will move vertically either upwardly or downwardly, from the median position shown and for which the shed is closed. The stroke of each frame a may, at the desired moment, thereby be reduced by half with respect to conventional open-shed dobbies.
Nonetheless, it should be observed that the structure of the known heavy dobbies, as recalled hereinabove, present considerable drawbacks in practice. In particular, it will be readily appreciated that the knives g must stop for a period of time when they arrive in the median position so that the device f can cause hook e to swing in one direction or the other. Such stopping obviously results in a substantial reduction in the operational speed of the dobby and of the loom, and at the same time creates detrimental effects of acceleration and deceleration.
U.S. Pat. No. 3,612,108 to STAUBLI in particular proposes dobbies capable of operating with open shed, closed shed or mixed shed, in which the traction levers controlled by the actuation elements depending on the weaving program, are connected to the drawing system of each heddle frame via a pivoting arm. The arm is associated with additional actuation elements adapted to impart to the arm a periodic swinging movement in synchronism with the control cycle of the actuation elements.
In that Patent, the intermediate pivoting arm is pivotally mounted on a fixed shaft while the additional actuation elements include a cam mechanism. Against the periphery of this cam is elastically applied a follower roller carried by the pivoting arm, which is connected to the corresponding traction lever by a slide system.
Operation obtained with such a structure is not entirely satisfactory and the user encounters considerable drawbacks in practice. In particular, control of the swinging or pivoting arm is not positively ensured as the cam mechanism can give only a lifting movement to the corresponding heddle frame.
In other types of dobbies capable of operating with open shed and closed shed, the actuation elements are connected in twos so that, in each pair, one of the elements ensures normal control of the traction lever of the pair while the other is adapted to give the pivoting arm coupled to the lever an additional cyclic movement corresponding to the closed shed. Reference be made on this point to U.S. Pat. Nos. 3,759,298 and 4,412,563 to STAUBLI.
Such a system indeed allows a positive control to be obtained, but the necessity of coupling the actuation elements in twos obviously reduces by half the number of heddle frames capable of being controlled by the dobby, for the same dimensions thereof.
It is an object of the present invention to overcome all the different types of drawbacks mentioned above.